South Africa's main opposition Democratic Alliance has signed a merger agreement with fellow opposition party the Independent Democrats.

The agreement between Democratic Alliance (DA) leader Helen Zille and Independent Democrats (ID) president Patricia de Lille aims at cutting into the dominance of the ruling African National Congress (ANC) ahead of next year's local races and national polls in 2014.

The ANC won 66 per cent of the vote in national elections last year and currently holds roughly two-thirds of seats in local government. The DA won 17 per cent of the vote and the ID one percent in last year's polls.

Ms Zille, who is also premier of the DA's stronghold Western Cape province, said the merger would allow the opposition to contend at a national level and hinted it would challenge the ruling party's dominance of the majority black vote, a legacy of the ANC's role in the anti-apartheid struggle.

"This step takes us closer to building a new majority that can win elections across South Africa," she said.

"This memorandum of the DA and the ID heralds the beginning of a new phase in our journey away from the politics of racial identity, towards the politics of shared values."

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The parties agreed to contest elections under the DA's banner, with "an appropriate number" of ID members joining the DA's top leadership committees.

Political analysts said the merger was unlikely to have a major short-term impact on the political landscape.

"I don't think it will have significant impact on their total vote count at this stage, but I do think that for the local government elections coming up next year... there might be some small gains there for the DA," Ivor Jenkins, director of the Institute for Democracy in South Africa, said.